Musculoskeletal injuries have an enormous impact on quality of life and remain one of the leading reasons that patients seek medical care. Engineered tissue grafts have the potential to repair damaged tissues when traditional transplants are unavailable or fail. Our laboratory has developed a novel emulsion templating methodology to generate microcellular polymer scaffolds for bone regeneration. A significant advance by our laboratory is the translation of this technology to tissue engineering through the development of injectable, high porosity bone grafts. Injectable scaffolds that cure in situ can fil irregular shaped defects, improve contact between the scaffold and surrounding tissue, and eliminate the need for costly molding techniques.13,14 Emulsion templating has several advantages over current injectable materials that suffer from low porosity and biodegradability (e.g., bone cements) or inability to withstand physiological loading (e.g., hydrogels). In the current proposal, we will develop a second generation polyHIPE that utilize hydroxyapatite (HA) nanoparticles to impart osteoiductive character to the bone graft. We will also investigate the potential of cell encapsulation in the HIPEs prior to cure as a means to deliver and retain MSCs at the defect site. Specific Aim 1: Develop and characterize osteoblastic differentiation on polyHIPE scaffolds containing HA nanoparticles Specific Aim 2: Evaluate key deployment variables of injectable polyHIPE scaffolds and viability of hMSCs after encapsulation. Following successful completion of this R21 project, the proposed grafts will be a significant advance in bone grafting procedures by providing a highly porous scaffold that 1) space-fills irregular shaped defects to promote superior tissue integration; 2) cures to suitable mechanical strength; 3) delivers hMSCs directly to the defect site; and 4) provides the necessary cues for osteogenic differentiation of those hMSCs. Subsequent investigation in an R01 will examine the potential of these osteoinductive grafts to enhance regeneration in critical size bone defects and provide expanded mechanistic studies of osteogenesis and vascularization in large bone grafts.